Google today released Chrome 39, the company's first 64-bit browser for OS X from its "stable" branch.

The update also patched 42 vulnerabilities and paid $41,500 in bounties to the bug hunters who reported a dozen of the flaws.

Chrome 39's most visible change was on the Mac, where it shifted from a long-available 32-bit browser to one designed and optimized for 64-bit PCs and operating systems.

In August, Google announced that Chrome 38 would be the first in its series of 64-bit browsers on OS X. But the next month Google revised the schedule. "We're now bringing these benefits to OS X with Chrome 64-bit for Mac, version 39, due to be released in November," the company said in a brief blog update on Sept. 12.

Google has touted Chrome 64-bit on OS X as faster to launch and less of a memory glutton than the older 32-bit edition.

The appearance today of Chrome 64-bit also signaled the retirement of the 32-bit version on the Mac, which will be stuck on Chrome 38.

Apple's then-CEO Steve Jobs trumpeted the switch from the PowerPC line of processors to Intel CPUs in early 2005, and his company began selling Intel-based Macs in January 2006. The first generations relied on 32-bit processors, but by August 2007, Apple had completed the transition to 64-bit.

That means Chrome users with older Intel-based Macs -- those sold between January 2006 and August 2007, at the latest -- will not be able to run the new browser.

Apple shifted its lines from 32- to 64-bit at different times: The MacBook Pro, for instance, went 64-bit in October 2006, while the less-expensive MacBook switched to 64-bit in November 2006. Apple stopped selling 32-bit iMacs in September 2006, and shifted to 64-bit for the Mac Mini in August 2007.

The MacBook Air has always featured 64-bit processors.

OS X 10.6, aka Snow Leopard, which was released in August 2009, was the last edition to support 32-bit Intel Macs.

Owners of older Intel 32-bit Macs still running Snow Leopard -- or even earlier versions, like 2007's Leopard or 2005's Tiger -- will have to either risk running an unpatched Chrome 38 or switch to a different browser. Mozilla's Firefox, for example, includes both 32- and 64-bit versions in each OS X edition's package. Opera Software's Opera also comes in a 32-bit edition.

According to Internet metrics firm Net Applications, just under 13% of all Macs ran Snow Leopard, Leopard or an earlier OS last month. A portion of those machines, however, will be capable of running Chrome, as they boast 64-bit CPUs, even though they're powered by out-of-date operating systems.

Of the 42 vulnerabilities patched in Chrome 39, 12 qualified for bug bounties, which ranged from a high of $16,500 split four ways to a handful of $500 rewards paid to individual researchers.

Last month, Google tripled the maximum bounty for bug reports from $5,000 to $15,000. Its top award for Chrome 39's collection was $7,500 for a "double free" flaw in Adobe's Flash, which is packaged with Chrome.

Eleven of the 12 that Google called out in a short advisory were rated "high," the company's second-most-severe threat level.

Chrome 39 can be downloaded from Google's website. Existing users will automatically be updated to the newest version.

As of v. 39, Chrome is now 64-bit only on the Mac.

This story, "Chrome for OS X Turns 64-bit, Forsakes Early Intel Macs" was originally published by
Computerworld.